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I'd recommend using grabicon.com instead of google's S2 favicon service. Google's service was never meant to be public, and could go away at any time. Grabicon was built to be a drop-in replacement - it's also free, and it works the same way.

It also lets you specify what size you want the icon to be, and generates unique default icons for sites that don't have one. This ensures a uniform user experience for web/mobile apps, because icons are always the requested size, and never missing.